The distance between optical terminals of optical fiber transmission systems is limited by the optical power that can be launched into optical fiber by optical transmitters of the optical terminals, the loss and dispersion of optical fiber interconnecting the optical terminals, and the sensitivity of optical receivers of the optical terminals. Where the distance between desired end points of an optical fiber transmission system exceeds the maximum distance between optical terminals, optoelectronic repeaters have been provided. Each optoelectronic repeater comprises an optical receiver for converting the optical signal to an electrical signal, electronics for regenerating the electrical signal, and an optical transmitter for converting the regenerated electrical signal to an optical signal for transmission to the next optoelectronic repeater or to a terminal of the system.
In Wavelength Division Multiplexed (WDM) optical fiber transmission systems which use optoelectronic repeaters, the optical signals are optically demultiplexed at each repeater, so that the signal at each distinct wavelength is coupled to a respective optical receiver for conversion to a respective electrical signal, each respective electrical signal is separately regenerated, each regenerated signal is applied to a respective optical transmitter operating at a distinct wavelength, and the transmitted signals are optically multiplexed for transmission to the next optoelectronic repeater or to a terminal of the system.
The performance of optoelectronic repeaters must be monitored so that faults in operation of the transmission system can be isolated to faulty repeaters or terminals, and maintenance personnel can be dispatched to appropriate locations with appropriate information and equipment to correct the faults. The performance of the optoelectronic repeaters is generally assessed by monitoring characteristics of the regenerated electrical signals at each repeater.
As the line rates of optical fiber transmission systems increase into the 2.5 Gbps to 10 Gbps range, higher speed electronics are needed in optoelectronic repeaters, and this increases the cost of optoelectronic repeaters.
Optical amplifiers, for example Erbium Doped Fiber Amplifiers (EDFAs), amplify optical signals directly without converting them to electrical signals. Because EDFAs do not require high speed regeneration electronics, they can be cheaper than optoelectronic repeaters for high speed optical fiber transmission systems.
Moreover, in WDM optical fiber transmission systems, the EDFAs can amplify optical signals at multiple wavelengths without optically demultiplexing them, thereby avoiding the costs of optical multiplexing and demultiplexing, and the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. Consequently, EDFAs can also be cheaper than optoelectronic repeaters for WDM systems.
As in systems using optoelectronic repeaters, performance of transmission systems using EDFAs must be monitored so that faults in the operation of the transmission systems can be isolated to faulty EDFAs or terminals, and maintenance personnel can be dispatched to appropriate locations with appropriate information and equipment to correct the faults. Because regenerated electrical signals are not available for monitoring at EDFAs, performance of EDFAs cannot be monitored using conventional optoelectronic repeater performance monitoring techniques.
Jensen et al disclose a method for monitoring performance of EDFAs in long haul undersea optical fiber transmission systems (R. A. Jensen et al, "Novel technique for monitoring long-haul undersea optical-amplifier systems", OFC '94 Technical Digest, Feb. 20, 1994). High speed optical signals are intensity modulated by a low frequency, low modulation index carrier, the carrier being modulated by a pseudorandom sequence. At each EDFA, a small portion of the optical signal is tapped by an optical coupler and coupled via a high loss optical loopback path to an optical transmission path carrying optical signals back to the terminal from which the optical signal originated. The optical signal received at the originating terminal is digitally correlated with appropriately delayed versions of the transmitted pseudorandom sequence to separate portions of the received signal which result from each optical loopback connection. The separated portions of the received signal are averaged over time to estimate the net gain or loss of the transmission paths to each of the EDFAs and back.
Unfortunately, some faults cannot be isolated using the net gain or loss estimates provided by the monitoring technique of Jensen et al. In particular, some faults degrade the performance of the optical fiber transmission systems by increasing optical noise generated at optical amplifiers rather than by decreasing the net gain of the optical amplifiers. The increased optical noise causes bit errors at terminals of the optical transmission system even though the received optical signal strength meets design objectives.
For example, variations of pump laser wavelength due to aging of pump lasers or due to malfunctions of pump laser temperature control devices can increase optical noise generated by EDFAs. Cabled fiber effects and changes in optical signal wavelength or optical signal strength can change spectral characteristics of optical noise, thereby changing amplification of optical noise relative to optical signals at EDFAs.
Excessive losses at the input end of EDFAs are also difficult to detect from net gain measurements, because such losses are generally compensated by higher gain toward the output end of the EDFAs. However, the compensating gain is applied to optical noise as well as to optical signals, so the net effect is a decreased signal to noise ratio at the output of the faulty EDFA.